Embodiments of the invention relate in general to optical devices which comprise semiconductor nanoparticles and in particular to lighting devices which include conversion layers having semiconductor quantum confined nanoparticles.
Light emitting diodes (LED) offer significant advantages over incandescent and fluorescent lamps with respect to their high energy efficiency and long lifetimes. LEDs are applicable in diverse applications including displays, automobile and signage lighting and domestic and street lighting.
A LED can emit monochromatic light in different regions of the spectrum, depending on the inorganic semiconductor compound used to fabricate it. However, “white” light, which is required for a very large portion of the lighting industry, cannot be generated using a conventional LED. Current solutions of producing white light include the use of three or more LEDs with various colours (e.g. Red, Green and Blue or “RGB”), or the use of a colour conversion layer of phosphor material (e.g. Cerium:YAG) to generate a broad white spectral emission from the ultraviolet (UV) or blue emission of a LED. However, such white light is almost always non-ideal and has in many cases undesired or unpleasant characteristics which may require improvement or correction.
Colloidal based semiconductor quantum dots (QD) offer the possibility of obtaining a colour gamut similar to and even better than the one obtained with the multi-LED solution, using the narrow-band emission of a QD tunable by size. Conversion layers incorporating ODs are known, see e.g. U.S. Pat. Nos. 7,264,527 and 7,645,397 and US patent applications 2008/0173886 and 2009/0162011. However, conversion layers based on QDs have challenges. These include for example losses due to re-absorption effects, whereby the QD emission is reabsorbed by other QDs in the layer. Generally this will occur for a red QD absorbing the emission emanating from QDs which emit more to the blue. This undesired process leads to reduced energy efficiency of a regular QD conversion layer and also to changes in the colour composition. The inherent size distribution of QD samples already provides different colours around a central colour. Therefore, re-absorption will take place inherently within such a layer. In devices where phosphor is used as part of a light conversion scheme to produce green light, the QD layers will absorb partially the light from the phosphor as well, leading to both re-absorption losses and colour changes.
In some cases, a close-packed conversion layer is desired. Close-packed QD conversion layers suffer from the phenomenon known as Fluorescence Resonant Energy Transfer (FRET), see e.g. Joseph R. Lakowicz, “Principles of Fluorescence Spectroscopy”, 2nd edition, Kluwer Academic/Plenum Publishers, New York, 1999, pp. 367-443. FRET occurs between a donor QD which emits at a shorter (e.g. bluer) wavelength relative to an acceptor QD positioned in close proximity and which emits at longer wavelength. There is a dipole-dipole interaction between the donor emission transition dipole moment and the acceptor absorption transition dipole moment. The efficiency of the FRET process depends on the spectral overlap of the absorption of the donor with the emission of the acceptor. The FRET distance between quantum dots is typically 10 nm or smaller. The efficiency of the FRET process is very sensitive to distance. FRET leads to colour change (red shift) and losses in the efficiency of light conversion.
Core/shell nanoparticles (NPs) are known. These are discrete nanoparticles characterized by a heterostructure in which a “core” of one type of material is covered by a “shell” of another material. In some cases, the shell is grown over the core which serves as a “seed”, the core/shell NP known then as a “seeded” NP or SNP. The term “seed” or “core” refers to the innermost semiconductor material contained in the heterostructure. FIG. 1 show schematic illustrations of known core/shell particles. FIG. 1A illustrates a QD in which a substantially spherical shell coats a symmetrically located and similarly spherical core. FIG. 1B illustrates a rod shaped (“nanorod”) SNP (RSNP) which has a core located asymmetrically within an elongated shell. The term nanorod refers to a nanocrystal having a rod-like shape, i.e. a nanocrystal formed by extended growth along a first (“length”) axis of the crystal with very small dimensions maintained along the other two axes. A nanorod has a very small (typically less than 10 nm) diameter and a length which may range from about 6 nm to about 500 nm.
Typically the core has a nearly spherical shape. However, cores of various shapes such as pseudo-pyramid, cube-octahedron and others can be used. Typical core diameters range from about 1 nm to about 20 nm. FIG. 1C illustrates a QD in which a substantially spherical shell coats a symmetrically located and similarly spherical core. The overall particle diameter is d2, much larger than the core diameter d1. The magnitude of d2 compared with d1 affects the optical absorbance of the core/shell NP.
As known, a SNP may include additional external shells which can provide better optical and chemical properties such as higher quantum yield (QY) and better durability. The combination may be tuned to provide emitting colours as required for the application. The length of the first shell can range in general between 10 nm and 200 nm and in particular between 15 nm and 160 nm. The thicknesses of the first shell in the other two dimensions (radial axis of the rod shape) may range between 1 nm and 10 nm. The thickness of additional shells may range in general between 0.3 nm to 20 nm and in particular between 0.5 nm to 10 nm.
In view of the numerous deficiencies of QD conversion layers mentioned above, there is a need for and it would be advantageous to have conversion layers which do not suffer from such deficiencies. In particular, there is a need for and it would be advantageous to have nanoparticle-based thin conversion layers with negligible re-absorption (of both same and different colour), negligible clustering and high-loading effects and negligible FRET.